In the related art, an NOx occlusion reduction type catalyst is known as a catalyst which reduces and purifies a nitrogen compound (NOx) in an exhaust gas discharged from an internal combustion engine. When the exhaust gas is under a lean atmosphere, the NOx occlusion reduction type catalyst occludes the NOx contained in the exhaust gas. When the exhaust gas is under a rich atmosphere, the NOx occlusion reduction type catalyst detoxifies the occluded NOx through reducing and purifying with hydrocarbon contained in the exhaust gas, and discharges the NOx. For this reason, in a case where the NOx occlusion amount of the catalyst reaches a predetermined amount, in order to recover NOx occlusion capacity, it is necessary to regularly perform the so-called NOx purge which makes the exhaust gas be in a rich state by a post injection or an exhaust pipe injection (for example, see JP-A-2008-202425).
On the other hand, a technique is proposed in an exhaust post-treatment device of a diesel engine which suppresses deterioration of an NOx purification rate even when a load variation arises immediately after the start of rich-spike treatment (for example, see JP-A-2014-125975).
A technique is proposed In the exhaust purification system that determines a deterioration state of an NOx purification catalyst from a detection value of an oxygen concentration sensor without using an NOx concentration sensor, thereby precisely setting a lean period until the start of the next regeneration control and preventing the decrease of the NOx purification rate (for example, see JP-A-2006-336518).